In current wavelength division multiplexing (WDM) optical communication systems, the tolerance of a nominal central frequency (e.g. +−1.5 GHz) of the laser at a transmitter of the WDM communication system is typically tolerable, because the ratio between the spacing (or bandwidth) of the optical communication channels (i.e. of the WDM channels) and the baud-rate per channel is relatively large. A typical channel spacing of DWDM (Dense WDM) systems is 50 GHz and typical baud-rates (e.g. using QPSK symbols) are at 28 Gbaud. Hence, typical spacing/baud-rate ratios are in the range of 1.8 or greater. Moreover, the frequency offset between the laser source at the transmitter and the local oscillator at a coherent receiver may typically be corrected by digital signal processing at the receiver. The digital signal processing for offset compensation can typically tolerate a pre-determined maximum frequency offset. For example, with QPSK symbols at 28 Gbaud (or Giga symbols/second) the maximum tolerable frequency offset with a channel spacing of 50 GHz is typically +/−3.5 GHz. In view of the above mentioned tolerances of laser sources in current coherent systems, frequency offsets between the laser source at the transmitter and the local oscillator at the coherent receiver may be compensated using digital signal processing.
On the other hand, in ultra-dense WDM systems, the channel spacing of the WDM channels is expected to be close to the symbol-rate (with a spacing/baud-rate ratio <1.2). Such ultra-dense WDM systems are expected to make use of narrow-band optical filters within the WDM channels (e.g. at the transmitter) in order to minimize or reduce the cross-talk between adjacent WDM channels. The narrow-band optical fibers are expected to provide a ratio bandwidth/baud-rate <1.2. In such ultra-dense WDM systems, the frequency stability of the laser source at the transmitter and in particular the alignment of the frequency of the laser source with respect to the central frequency of the narrow-band optical filters are expected to be a technical challenge.
As indicated above, the laser sources employed in optical communication systems typically have a tolerance of + or −1.5 GHz. These tolerances are too high with respect to the requirements of future optical systems (having a baud-rate in the range of >32.5 GBaud, and a channel spacing in the range of approx. 33.3 GHz). This is illustrated in the Q2-factor penalty vs. frequency offset diagram 100 of FIG. 1. It can be seen that in case of no or a wide-band optical filtering at the transmitter (reference numeral 101), the communication system supports relatively high offsets between the frequency at the transmitter and the local oscillator (LO) frequency at the receiver. On the other hand, in case of narrow-band optical with the WDM channel (as is the case in ultra-dense WDM systems), even small deviations of the frequencies at the transmitter and the receiver lead to significant bit error rates (reference numeral 102). Hence, when operating with a signal bandwidth close to the baud-rate, a relatively small mismatch between the central frequency of the transmitted signal and the central frequency of the optical filter translates into a significant penalty.
The present document addresses the above mentioned technical issues. In particular, the present document describes a method and a system for aligning the frequencies of the laser source (at the transmitter) and the local oscillator (at the receiver), in order to allow for the implementation of ultra-dense WDM systems which a reduced channel spacing/baud-rate ratio compared to present optical WDM communication systems.